Today’s Question: All my Lightroom Classic photos are on an external drive. My catalog is on the computer’s internal hard drive. Will the XMP sidecar be on the external drive with the photos or on the internal drive with the catalog?
Tim’s Quick Answer: The XMP sidecar files generated by Lightroom Classic will be stored alongside the original raw capture that the metadata relates to, which in this case means on the external hard drive where the photos are stored.
More Detail: In Lightroom Classic you can enable an option to write standard metadata updates to your source photos by turning on the “Automatically write changes into XMP” checkbox on the Metadata tab of the Catalog Settings dialog. With this option enabled standard metadata information as well as Develop module adjustments will be saved in an XMP “sidecar” file for raw captures. That information will be written directly into metadata for other supported image formats.
The key is that this information is saved in the same location as your photos, which may be a different location from the Lightroom Classic catalog. The source information is of course still contained within the catalog, but the point is that the metadata updates are saved in the location with your photos, not alongside the catalog files.
Keep in mind that this option to write metadata out to the photos themselves in addition to the catalog is not a replacement for making sure your catalog is backed up. Features specific to Lightroom Classic are not preserved with this option to write metadata to the photos. For example, collections, virtual copies, history, as well as pick and reject flags are only saved to the Lightroom Classic catalog and aren’t saved out to files even with the option to save metadata to photos enabled.